This invention relates in general to electrophotography and, more specifically, to an electrophotographic or xerographic imaging member having a charge transport layer containing a hole transport small molecule dispersed in a film forming binder, the layer be formed from a coating solution comprising a high boiling and a low boiling solvent. In the art of xerography, a xerographic plate comprising a photoconductive insulating layer is imaged by first uniformly depositing an electrostatic charge on the imaging surface of the xerographic plate and then exposing the plate to a pattern of activating electromagnetic radiation such as light which selectively dissipates the charge in the illuminated areas of the plate while leaving behind an electrostatic latent image in the non-illuminated areas. This electrostatic latent image may then be developed to form a visible image by depositing finely divided electroscopic marking particles on the imaging surface.
A photoconductive layer for use in xerography may be a homogeneous layer of a single material such as vitreous selenium or it may be a composite layer containing a photoconductor and another material. One type of composite photoconductive layer used in electrophotography is illustrated in U.S. Pat. No. 4,265,990. A photosensitive member is described in this patent having at least two electrically operative layers. One layer comprises a photoconductive layer which is capable of photogenerating holes and injecting the photogenerated holes into a contiguous charge transport layer. Generally, where the two electrically operative layers are positioned on an electrically conductive layer with the photoconductive layer sandwiched between a contiguous charge transport layer and the conductive layer, the outer surface of the charge transport layer is normally charged with a uniform electrostatic charge and the conductive layer is utilized as an electrode. In flexible electrophotographic imaging members, the electrode is normally a thin conductive coating supported on a thermoplastic resin web. Obviously, the conductive layer may also function as an electrode when the charge transport layer is sandwiched between the conductive layer and a photoconductive layer which is capable of photogenerating electrons or holes and injecting the photogenerated electrons or holes into the charge transport layer. The charge transport layer in this embodiment, of course, must be capable of supporting the injection of photogenerated electrons from the photoconductive layer and transporting the electrons through the charge transport layer.
Various combinations of materials for charge generating layers and charge transport layers have been investigated. For example, the photosensitive member described in U.S. Pat. No. 4,265,990 utilizes a charge generating layer in contiguous contact with a charge transport layer comprising a polycarbonate resin and one or more of certain aromatic amine compounds. Various generating layers comprising photoconductive materials exhibiting the capability of photogeneration of holes and injection of the holes into a charge transport layer have also been investigated. Typical photoconductive materials utilized in the generating layer include amorphous selenium, trigonal selenium, and selenium alloys such as selenium-tellurium, selenium-tellurium-arsenic, selenium-arsenic, and mixtures thereof. The charge generation layer may comprise a homogeneous photoconductive material or particulate photoconductive material dispersed in a binder. Other examples of homogeneous dispersions of conductive material in binder charge generation layer are disclosed in U.S. Pat. No. 4,265,990. Additional examples of binder materials such as poly(hydroxyether) resins are taught in U.S. Pat. No. 4,439,507. The disclosures of the aforesaid U.S. Pat. No. 4,265,990 and U.S. Pat. No. 4,439,507 are incorporated herein in their entirety. Photosensitive members having at least two electrically operative layers as disclosed above in, for example, U.S. Pat. No. 4,265,990, provide excellent images when charged with a uniform negative electrostatic charge, exposed to a light image and thereafter developed with finely developed electroscopic marking particles.
If a flat, biaxially oriented polyethylene terephthalate (e.g. 3 mil thick PET) sheet is solvent coated with an imaging layer, for example a solution of 50 percent by weight polycarbonate (e.g. Makrolon) and 50 percent by weight aromatic diamine dissolved in a solvent to form a charge transport layer (CTL) about 1 mil thick, the multilayer structure tends to curl upon drying. This is due to the dimensional contraction of the applied (CTL) coating relative to the PET substrate from the point in time when the applied (CTL) coating solidifies and adheres to the underlying surface. The solidification point is the glass transition temperature (Tg) of applied coating. Once this solidification point is reached, further evaporation of coating solvent and/or cooling below Tg causes continued shrinking of the applied coating layer due to volume contraction resulting from removal of additional solvent and/or differential thermal contraction will cause the coated sheet to curl toward the applied layer because the PET substrate undergoes smaller dimensional changes. This relative contraction occurs isotropically, i.e., in three-dimensions. In other words, from the point in time when the applied coating has reached the Tg and is anchored at the interface with the underlying support layer, continued shrinking of the applied coating causes dimensional decreases in the applied coating which in turn builds up internal tension stress in the two dimensions constrained by adhesion to the substrate and, therefore, forces the entire coated structure to curl toward the dried applied coating. If the coated article has a circular shape, the curled structure will resemble a bowl. If the Tg of the coated CTL layer is about 50 degrees C above the operating temperature of the imaging member the relative shrinkage is about 0.6%.
Curling is undesirable for several reasons. First, because many of the electrophotographic imaging process depend critically on the spacing between the component and the imaging member; any variation in the flatness adversely affect the quality of the ultimate developed images. For example, non-uniform charging distances may be manifested as variations in the electrostatic latent images. Also the built-in stress weakens the adhesion between the layers, leading to adhesion failures. Moreover, the additional stress combined with the stress from constant flexing of multilayered photoreceptor belts during cycling can cause stress cracks to form due to fatigue and an earlier failure. These cracks print out on the final electrophotographic copy. Premature failure due to fatigue prohibits use of these belts in designs utilizing small roller sizes (e.g. 20 mm or smaller) for effective auto paper stripping. Note that the stretching of the coated layer on a 20 mm roll is approximately equal to 0.6% hence the stress is twice what it would be without the built in stress. In other words, flexing a belt with a built in 0.5% shrinkage stress on a 20 mm roll is equivalent to flexing an unstressed belt around a 12 mm roll.
The curl can be counteracted by applying a coating to the underside of the imaging member, i.e. the side of the supporting substrate opposite the electrically active layer or layers. However, such coating requires an additional coating step on a side of the substrate opposite from the side where all the other coatings are applied. This additional coating operation normally requires that a substrate web be unrolled an additional time merely to apply the anticurl layer. Also, many of the solvents utilized to apply the anti-curl layer require additional steps and solvent recovery equipment to minimize solvent pollution of the atmosphere. Further, equipment required to apply the anti-curl coating must be cleaned with solvent and refurbished from time to time. The additional coating operations raise the cost of the photoreceptor, increase manufacturing time, and decrease production throughput. Also the extra coating decreases production yield by, for example, increased likelihood that the photoreceptor will be damaged by the additional handling. Furthermore, the anticurl coating does not eliminate the built in stress and the problems that it causes, such as premature failure with cycling. Also, other difficulties have been encountered with these anti-curl coatings. For example, photoreceptor curl can sometimes still be encountered due to a decrease in anticurl layer thickness resulting from wear in as few as 1,500 imaging cycles when the photoreceptor belt is exposed to stressful operating conditions of high temperature and high humidity. The curling of the photoreceptor is inherently caused by internal stress build-up in the electrically active layer or layers of the photoreceptor which promotes dynamic fatigue cracking, thereby shortening the mechanical life of the photoreceptor. Further, the anticurl coatings occasionally separate from the substrate during extended machine cycling and render the photoconductive imaging member unacceptable for forming quality images. Anticurl layers will also occasionally delaminate due to poor adhesion to the supporting substrate. Moreover, in electrophotographic imaging systems where transparency of the substrate and anticurl layer are necessary for rear exposure erase to activating electromagnetic radiation, any reduction of transparency due to the presence of an anticurl layer will cause a reduction in performance of the photoconductive imaging member. Although the reduction in transparency may in some cases be compensated by increasing the intensity of the electromagnetic radiation, such increase is generally undesirable due to the amount of heat generated as well as the greater costs necessary to achieve higher intensity.
Further, the built in mechanical stresses which, when perturbed by wear, results in distortions which resemble ripples. These ripples are the most serious photoreceptor related problem in advanced precision imaging machines which demand precise tolerances. When ripples are present, different segments of the imaging surface of the photoconductive member are located at different distances from charging devices, developer applicators, toner image receiving members and the like during the electrophotographic imaging process thereby adversely affecting the quality of the ultimate developed images. For example, non-uniform charging distances can be manifested as variations in high background deposits during development of electrostatic latent images. It is theorized that since the anticurl backing layer is usually composed of material that is less wear resistant than the adjacent substrate layer, it wears rapidly during extended image cycling, particularly when supported by stationary skid plates. This wear is nonuniform and leads to the distortions which resemble ripples and also produces debris which can form undesirable deposits on sensitive optics, corotron wires and the like.
Another property of significance in multilayer devices is the charge carrier mobility in the transport layer. Charge carrier mobilities determine the velocities at which the photoinjected carriers transit the transport layer. To achieve maximum discharge or sensitivity for a fixed exposure, the photoinjected carriers must transit the transport layer before the imagewise exposed region of the photoreceptor arrives at the development station. To the extent the carriers are still in transit when the exposed segment of the photoreceptor arrives at the development station, the discharge is reduced and hence the contrast potentials available for development are also reduced. For greater charge carrier mobility capabilities, it is normally necessary to increase the concentration of the active small molecule transport compound dissolved or molecularly dispersed in the binder. Phase separation or crystallization sets an upper limit to the concentration of the transport molecules that can be dispersed in a binder. One way of increasing the solubility limit of the transport molecule is to attach long alkyl groups on to the transport molecules. However, these alkyl groups are "inactive" and do not transport charge. For a given concentration of the transport molecules, these side chains actually reduce the charge carrier mobility. A second factor that reduces the charge carrier mobilities is the dipole content of the charge transport molecules, their side groups as well as that of the binder in which the molecules are dispersed. One of the prior art of reducing the curl involves an imaging member comprising hole transporting material containing at least two long chain alkyl carboxylate groups dissolved or molecularly dispersed in a film forming binder. Prior art suggests the use of these molecules containing long chain alkyl carboxylate groups dispersed in a binder or in combination with a conventional hole transport molecule. However, when in combination with the conventional transport molecule, the concentration of the molecule with the long alkyl carboxylate groups has to be considerably larger than 15 weight percent in order to eliminate curl. Although curl is eliminated and these devices can be used in electrophotography, high speed electrophotography requires high charge carrier mobilities. Use of a large concentration of transporting material containing at least two long chain alkyl carboxylate groups results in a drop in mobilities because of the "inactive" long chains required to reduce curl as well as the high dipole content of these long alkyl carboxylate groups.